Diode-laser light is commonly used for optically pumping solid-state lasers and fiber lasers. As light from a single diode-laser is often insufficiently powerful for such pumping, it is usual to use light from a plurality of diode-lasers arranged in a linear array. Such an array is commonly referred to by practitioners of the art as a diode-laser bar. The light from the diode-lasers in the bar must be collected by an optical arrangement that makes the sum of the outputs of the diode-lasers available for pumping.
Light is emitted from a diode-laser as a diverging beam. The beam diverges strongly, for example at about 35 degrees, in one axis, termed the fast axis, and diverges weakly, for example at about 10 degrees, in an axis (the slow axis) perpendicular to the fast axis. In a diode-laser bar individual diode-lasers (emitters) are arranged, spaced apart, linearly, in the slow axis direction. In a high power diode-laser of the type used for optical pump light is emitted in a plurality of modes (multimode output). A preferred method of collecting the outputs of the plurality of emitters of a diode-laser bar is to couple the individual emitter outputs into a corresponding plurality of multimode optical fibers having entrance ends thereof arranged in a linear array aligned with the slow axis of the diode laser bar. A cylindrical microlens is used to collimate the emitter output in the fast axis the output is coupled into the fibers. Output ends of the optical fibers are formed into a bundle. Light output from the bundle can be collected by a lens and focused directly into a solid-state gain medium or into a single optical fiber, which can be a transport fiber or a fiber laser to be pumped.
In the gain medium of a solid-state laser or a fiber laser absorption of optical pump light can often only occur in a narrow band of wavelengths, for example, about 1 nanometer (nm) wide. This narrow band of wavelengths is centered on a fixed, peak absorption wavelength that is characteristic of the gain medium. Absent any constraint, diode-laser light is emitted in a relatively broad spectrum of wavelengths, for example between 2 nm and 5 nm. Accordingly, for optimizing optical pumping efficiency, it is preferable to provide a constraint that narrows the emission bandwidth and stabilizes the center wavelength of this narrowed emission bandwidth at the characteristic peak absorption wavelength of the gain medium.
One arrangement that has been used to narrow the bandwidth and stabilize the wavelength of the output of a single mode diode-laser coupled into the core of a single mode fiber, is to write a fiber Bragg grating into the core of the fiber. The refractive index modulation and the period of modulation of the Bragg grating are selected such that the grating reflects back into the diode-laser a few percent (usually less than 10%) of radiation propagating in the core in a bandwidth less than about 1 nm about a peak-reflection wavelength determined by modulation period. This forces the diode laser to emit at the peak-reflection wavelength of the grating and with a bandwidth about equal to the reflection bandwidth of the grating.
This method however is not suitable for use with multimode fibers since a multimode fiber supports all the directions (angles) of propagation within its numerical aperture. The peak reflection wavelength of a fiber Bragg grating depends not only on the modulation period but on the angle of incidence of light on the grating. Accordingly, in a multimode fiber core, a Bragg grating would have a reflection bandwidth broadened by the plurality of angles at which the multimode light was incident thereon. The grating would provide neither adequate bandwidth narrowing nor adequate wavelength stabilization. There is a need for an arrangement for stabilizing the wavelength and narrowing the bandwidth of the output of a multimode fiber coupled, multimode diode-laser bar.